Add description to hint if conn_type is missing (#13778)

- add plaintext description to add/edit conn_type to make sure people remember to install necessary provider packages
diff --git a/airflow/www/views.py b/airflow/www/views.py
index 1d9a8ba..c488f34 100644
--- a/airflow/www/views.py
+++ b/airflow/www/views.py
@@ -2810,6 +2810,10 @@
         choices=sorted(_get_connection_types(), key=itemgetter(1)),
         widget=Select2Widget(),
         validators=[InputRequired()],
+        description="""
+            Conn Type missing?
+            Make sure you've installed the corresponding Airflow Provider Package.
+        """,
     )
     for key, value in ProvidersManager().connection_form_widgets.items():
         setattr(ConnectionForm, key, value.field)